1. Field of the Invention
This invention relates to a semiconductor optical ice, such as a laser which is optical frequency tunable.
2. Description of the Related Art
Optical frequency tunable lasers are key devices for coherent optical frequency division multiplexed (FDM) networks. Rapid tunability and narrow spectral linewidth are essential for such tunable lasers.
Distributed feedback (DFB) lasers are attractive for such system because of their narrow spectral linewidth during optical frequency tuning. A DFB laser has a grating to constitute a cavity in an active region where radiation occurs.
A frequency drift is caused by a thermal effect and plasma effect. The frequency drift is caused by a change of the refractivity of the cavity. The frequency drift caused by the change of the carrier density is known as a plasma effect. The density of carriers in the cavity is increased with the driving current. The increase in the carrier density causes the refractivity of the cavity to decrease. As a result, an oscillation frequency becomes high. In other words, the oscillation frequency is shifted to a short wavelength side, i.e. a blue shift. The amount of the plasma frequency shift in a conventional DFB laser is about -1 pm/mA and the response time of that is about 100 ps.
On the other hand, the temperature of the cavity becomes high with increasing driving current. The temperature rise causes the refractivity of the cavity to increase. As a result, an oscillation frequency becomes low. In other words, the frequency is shifted to a long wavelength side, i.e., a red shift. The frequency shift caused by the change of the temperature is known as a thermal effect. The amount of the thermal frequency shift in a conventional DFB laser is about +6 pm/mA and the response time is about 100 ms. The thermal frequency shift is slower, so we call it drift, and is larger than the plasma frequency shift.
The total drift is the sum of the thermal frequency drift and the plasma frequency drift. When the driving current is increased, first, the frequency will shift to a short wavelength side in 100 ps, then the frequency will drift to a long wavelength side across an initial wavelength and the drift will stop after 100 ms. In a DFB laser, the thermal frequency drift reverses the initial frequency shift due to the plasma effect and becomes dominant. The step response of the frequency is too complicated, 0(initial value).fwdarw.(+).fwdarw.0.fwdarw.(-). Therefore, a feedback control can not be used.
Meanwhile, a multi-electrode distribution Bragg reflector type (DBR) laser may have a large width of variable oscillation wavelength. The multi-electrode DBR laser includes a phase alignment region and a Bragg reflector region in addition to an active layer. These regions form a passive waveguide path in the cavity direction, wherein radiation does not occur. Electrodes are provided for each region. The injected currents to each region are controlled so as to change the refractivity of each region. As a result, frequency variable operation is performed.
In the passive waveguide path, since the carrier density is largely changed by the injected current, the plasma effect is predominant. As a result, in a multi-electrode DBR laser, the frequency can be quickly changed, because the response time of a carrier density change is short, measured in nanoseconds.
However, a spectral linewidth would be broadened to over 10 MHz during the optical frequency tuning, because of a recombination of carriers injected in the passive waveguide path. Therefore, it is considered that a DBR laser is not suitable for a coherent optical transmission.